Side-fed leaky-wave planer antennas have a feed waveguide which couples RF energy via slots to a slab-line consisting of a dielectric slab backed by a metal ground plane. The preferred mode is designated TM.sub.01. The TM.sub.01 mode has a component of electric field at the surface of the dielectric in the direction of propagation. Radiating elements are spread one wavelength apart across the slab. As a consequence of this spacing and the spacing of feed slots of the feed waveguide, at one frequency all the radiating elements are in phase and the resulting beam is broadside to the array. With a side fed antenna, when the operating frequency is changed, the beam will scan as the phase relationship changes between radiating elements. For communications applications it is desirable to have a beam that is fixed in direction if the operating frequency is scanned over a band.
In U.S. Pat. No. 5,005,022, Blaisdell disclosed that a solution to the beam scanning problem is to feed RF energy from the center of a circular leaky-wave antenna. The electrical length from the center to each point equidistant from the center will remain the same with change of operating frequency. The antenna may be regarded as being made up of pairs of radiating elements equidistant and opposite one another. Radiation from diametrically opposed radiating elements will be in phase since they are one guide wavelength apart. The radiating field is an on-axis broadside beam and remains on axis as the frequency changes since the electrical length to paired radiating elements will remain equal.
Blaisdell calls for a transition from circular waveguide to surface wave on a dielectric slab. It is desired to provide a center-fed leaky wave antenna having an improved transition from circular waveguide to surface wave on a dielectric slab.